Backflushable filtering apparatus for molten material and distribution unit for a filter device of this type

ABSTRACT

A backflushable filtering apparatus for a molten material, particularly for a plastic melt, comprises a housing ( 1 ) in which a sieving arrangement ( 17 ) is provided which includes at least two sieving sections ( 16 ) separated from one another. The melt to be filtered is supplied to the sieving sections ( 16 ) through at least one distributor ( 3 ). The distributor ( 3 ) includes at least one control body ( 9 ) for backflushing which is moveable within a housing ( 53 ) that is provided with an inlet opening ( 4 ) for the material to be filtered. The control body ( 9 ), in a filtering position, unblocks the influx of material to be filtered to all sieving sections ( 16 ) through connection channels ( 23 ). However, in a backflushing position, the control body ( 9 ) interrupts the influx of material to the filtering section ( 16 ) to be backflushed and interconnects the connection channel ( 23 ) of the sieving section ( 16 ) flushed back with a discharge channel ( 28 ) located in the control body ( 9 ). At least the majority of the circumference of the control body ( 9 ) is surrounded by a distribution space ( 7 ) for the material to be filtered which is situated within its housing ( 53 ). The distribution space ( 7 ) is connectable through the connection channels ( 23 ) to all sieving sections ( 16 ) in communication with the distributor ( 3 ). Guiding channels ( 6 ) lead from the inlet opening ( 4 ) to the regions of the two front ends ( 61, 62 ) of the distribution space ( 7 ). These front ends ( 61, 62 ) are situated in the region of the outermost connection channels ( 23 ) or outside the region of all connection channels ( 23 ). The discharge channel ( 28 ), which leads away from the control body ( 9 ), may be caused to communicate with the connection channel ( 23 ) of the respective sieving section ( 16 ) to be flushed back through a flush back channel ( 27 ) of at least one cross-piece ( 14 ) of the control body ( 9 ) which overbridges the distribution space ( 7 ).

The invention relates to a backflushable filtering apparatus for amolten material, particularly for a plastic melt, comprising a housingin which a sieving arrangement is provided which includes at least twosieving sections separated from one another, to which the melt to befiltered is supplied via at least one distributor which includes atleast one control body for backflushing, moveable within a housing thatis provided with an inlet opening for the material to be filtered, saidcontrol body, in a filtering position, unblocking the influx of materialto be filtered to all sieving sections through connection channels,while in a backflushing position interrupting the influx of material tothe filtering section to be backflushed and interconnecting theconnection channel of the sieving section flushed back with a dischargechannel located in the control body and leading away from it, at leastthe majority of the circumference of the control body being surroundedby a distribution space for the material to be filtered which issituated within the housing of the distributor, said distribution spacebeing connectable through the connection channels to all sievingsections in communication with the distributor. Furthermore, theinvention relates to a distributor for such a filtering apparatus.

A filter apparatus of the initially described kind which can beback-flushed, has become known (EP 1245366 A2, DE 19730574 C1). Withinthese constructions, the control body is constituted by a control pistonthat can be shifted within its housing in longitudinal direction and hasa central peripheral groove which is in flow connection with the inletopening and via which the material to be filtered is supplied either totwo piston-shaped sieve carriers or, if it has to be back-flushed, onlyto one of these sieve carriers. In the back-flushing position a conduitthat leads to the central discharge channel of the control body is inconnection with the downstream side of the back-flushed sieve. Withinthis, the expenditure caused by two separated sieve carrier pistons isof disadvantage as well as the reason that such a construction islimited to only two sieves. Further, material to be filtered may stayfor a longer time within the peripheral groove of the control body sothat there is the danger of coking of this material.

Within another back-flushable filter apparatus (AT 407611 B), thecontrol-body is constituted by a rotatable piston bearingly supportedfor rotation within its housing, which piston is intersected crosswiseto its axis by two radial through-passage conduits which in thefiltering position connect the inlet opening with two sieve section ofthe sieving arrangement. Each one of these sieve sections is formed by afilter element disposed within the housing of the filter device. Byrotation of the rotational piston around its longitudinal axis one ofthe two through-passage conduits can be so rotated that the flow of thematerial to be filtered to the associated filter is blocked, howeverthis filter which has to be back-flushed is connected with a dischargechannel for impurities which have to be carried off the back-flushedfilter, which discharge channel is disposed within the rotatable piston.Within this, it is of disadvantage within the temporary blocking of thesupply conduit during the back-flushing process that the materialpositioned within the conducting conduit leading from the inlet openingto the rotatable piston stands still and therefore tends to coking.Further, it is difficult to extend such a filter apparatus to more thantwo sieve sections.

It is an object of the invention to improve a filtering apparatus of thekind mentioned at the outset so that a continuous flow through allchannels is ensured which lead to the sieving sections during thefiltering procedure, thus avoiding coking of the material to befiltered, on the one hand, while the apparatus may be enlarged to anynumber of sieving sections desired without any problem, on the otherhand. This object is achieved according to the invention in that guidingchannels lead from the inlet opening to the regions of the two frontends of the distribution space, that said front ends are situated in theregion of the outermost connection channels or outside the region of allconnection channels, and that the discharge channel may be caused tocommunicate with the connection channel of the respective sievingsection to be flushed back through a flush back channel of at least onecross-piece of the control body which overbridges the distributionspace. In this way, the flow to the connection channels is alwaysdirected from the exterior to the middle so that no dead corners willexist in which plastic material could remain during the filteringprocedure, thus affecting its quality. A distribution space with such aflow will continuously be flown through by the material to be filtered,independently of whether a sieving section is just backflushed or not.

By arranging a plurality of cross-pieces off-set relative to each other,this construction may be enlarged to a high number of sieving sectionwithout any problem, nevertheless always ensuring backflushing sectionby sections. It is also advantageous that the sieving section flushedback has not necessarily to be moved during flushing back procedure sothat a sieve carrier, commonly used for holding a sieving section, doesnot wear off by the back flushing procedure. The sieve carrier, beingstationary during backflushing procedure, will therefore not introduceburnt or decomposed parts of melt into the melt that has already beenfiltered. Moreover, the apparatus may also be operated in an easiermanner, because, by simply moving the control body into a positiondifferent from the previous position, one sieve section after the othermay be blocked against the flow of melt, while concurrently connectingit with the discharge channel, independently of the number of existingsieving sections.

Within the scope of the constructive possibilities of realization, thereare two basic variants: the control body may be a slider displaceable inthe housing, that surrounds it, the connection channels leading to theindividual sieving sections being offset relative to each other insliding direction of the slider, on the one hand. On the other hand, thecontrol body may be a rotary piston, rotatable about its longitudinalaxis within its housing, which is supported in a sealed manner at itstwo front ends in its housing, but is surrounded in-between by thedistribution space. Though the former construction requires more spacedue to the longitudinal sliding motion of the slider, it istheoretically unlimited with respect to the number of sieving sections,while the latter construction is more space-saving.

According to a preferred embodiment of the invention, a particularlyadvantageous construction consists in that the connection channelsextend only in the housing, which surrounds the control body, and thatsieve nests of the housing of the filtering apparatus, which contain thesieving sections, immediately join the outer surface of the housing ofthe control body within the region of the respective orifice of aconnection channel. This results in a very short path between thedistribution space and the individual sieving sections. The consequenceis that, having terminated the backflushing procedure, the dirty flushback mass existing in the above-mentioned path is as small as possible,thus not affecting substantially the filter function of thecorresponding sieving section in the following filtering procedure.

It is especially favorable if, according to the invention, the sievingarrangement comprises a plurality of sieving sections arranged in atleast one array, the orientation of which being parallel to thelongitudinal direction of the distribution space. These numerous sievingsections separated from one another can each separately be flushed back,and, in the case of a suitable arrangement, can be flowed throughcentrally in relation to the connection channels, and therefore also cancentrally be flushed back, which enhances the efficiency both of thefiltering performance and of backflushing.

It is convenient to form each sieving station with a perforated supportplate and a perforated backflushing plate, at least one sieving layerbeing situated in-between. Both plates, as mentioned above, support thesieving layer during filtering procedure and the backflushing procedure,and prevent in this way any deformation of the sieving layer. Such aconstruction can easily be formed in such a manner that at least onesieving section is separated from the adjacent sieving section by apartition arranged upstream in filtering position, and a connectionchannel leads to each one of the sieving sections. This results in anincrease of the number of sieving sections being independent from oneanother, and therefore an improvement of the entire filteringperformance. The perforated support plate and the perforatedbackflushing plate may be made arcuate, the convex side being upstreamin filtering position. This results in an enlargement of the sievingsurface, and the resulting sieving space corresponds better to theoccurring flow conditions. At least one sieving section may comprise arectangular or square sieving surface, optionally with rounded corners,which results in an increase of the active filtering surface that isflown through in a uniform manner.

In order to favour splitting up the supplied melt fluid to centrallydisposed sieving sections, at least one additional conducting conduitmay end in the distribution space between the two conducting conduitsleading to the front ends of the distribution space.

Further, it is possible to provide at least two control bodies within acommon housing, each one of which is supplied in the region of the frontends of its distribution space via conducting channels. This maycontribute to enhance the uniformity of supplying the sieving sections.Further, this facilitates the common backflushing of more than onesieving section. Further, thereby the backflushing distances for thescreened-off impurity particles to be backflushed are made even shorter,and the material demand for backflushing is decreased.

The distributor for a backflushable filtering apparatus according to theinvention is based on a construction which comprises a housing and acontrol body for backflushing moveable therein, to which the material tobe filtered is supplied through an inlet opening and which, in filteringposition, unblocks the influx of this material to at least twoconnection channels which may be interconnected to sieving sections ofthe filtering apparatus, whereas in a flush back position, the controlbody interconnects one of these connection channels with a dischargechannel situated in the control body, the inlet opening communicatingwith a distribution space arranged in the housing and whose majoritysurrounds the control body. On this base, the construction according tothe invention of this distributor is characterized in that the inletopening communicates with the distribution space via two guidingchannels which discharge at its front end into the distribution spacewithin the region of the outermost connection channels or outside theregion of all connection channels. As has been mentioned above, thisresults in the advantage that the flow to the connection channels isalways directed from the exterior to the middle, and dead corners areavoided in which material to be filtered could remain for a long period.

At least one additional conducting conduit may lead to the distributionspace between the two conducting channels leading to the front ends ofthe distribution space.

Further characteristics and advantages of the invention will becomeapparent from the description of embodiments schematically illustratedin the drawings.

FIG. 1 shows a cross-section of a first embodiment having a singledistributor formed as a rotary piston.

FIG. 2 is a cross-section along the line II-II of FIG. 1.

FIG. 3 shows an embodiment having two distributors as a cross-sectionsimilar to that of FIG. 1, and

FIG. 4 is a cross-section along the line IV-IV of FIG. 3.

FIG. 5 shows an embodiment having two distributors formed as sliders ina cross-sectional view similar to that of FIG. 3, the two distributorsbeing in filtering position.

FIG. 6 shows a cross-section similar to that of FIG. 5 wherein, however,the left-side distributor is in flush back position.

FIG. 7 shows a cross-section of an embodiment having two sievingarrangements, each of which being sub-divided in partial sievingsurfaces.

FIG. 8 shows a cross-section, similar to that of FIG. 7, of a varianthaving a single sieving arrangement which is sub-divided into fivepartial sieving surfaces which may be flushed back independently fromone another.

FIG. 9 shows a cross-section along the line IX-IX of FIG. 8.

FIG. 10 shows an embodiment comprising partial sieving surfaces whichare formed as a rectangle.

FIG. 11 shows schematically an embodiment wherein a single distributoris associated to two opposite sieving arrangements.

FIG. 12 shows schematically the combination of a filtering apparatus anda device for plastifying and agglomerating plastic material.

FIG. 13 shows an embodiment having additional guide channels.

FIG. 14 shows an embodiment having two control bodies.

The embodiment of FIGS. 1 and 2 comprises a housing 1 for a sievecarrier 2 as well as a housing 53 for a distributor 3 associated to it.The housing 53 comprises an inlet opening 4 for the influx of a melt tobe filtered which is particularly formed by a molten thermoplasticmaterial. The material flowing in the direction of arrow 5 into theinlet opening 4 reaches two guide channels 6 leading to both front endsof a distribution space 7 which surrounds the majority of thecircumference of a control body 9 of the distributor 3 formed as arotary piston 8. This distribution space 7, at both its front ends 61,62, is closed by cylindrical portions 10, 11 of the rotary piston 8 withwhich the rotary piston 8 is supported in a borehole 12 of the housing 1so as to be rotatable about its longitudinal axis, but sealed. Thecontrol body 9 comprises between the two cylindrical portions 10, 11 aplurality of cross-pieces 14 extending through the distribution space 7which are offset relative to each other in circumferential direction ofthe distribution space, each of which being formed by a knob-likesalient 54 of the outer surface of the control body 9. The surface ofeach cross-piece 14 or of the salient 54 forming it which faces the wallof the borehole 12 is adapted to the shape of the borehole 12 so thatthe cross-piece 14 sealingly engages an area of the wall of the borehole12.

The sieve carrier 2 is formed as a cylindrical piston arranged in aborehole 15 of the housing 1. At the side facing the distributor 3, thesieve carrier 2 carries a plurality of sieving sections 16 arrangedside-by-side in its longitudinal direction, a single one of them beingrepresented in FIG. 2 for the sake of simplicity. These sieving sections16 together form a sieving arrangement 17 for filtering the materialsupplied to it. The individual sieving sections 16 are separated fromone another by partitions 18 arranged upstream of each sievingarrangement 17 and extending substantially perpendicularly to thelongitudinal axis 19 of the sieve carrier 2. Each sieving section 16comprises a perforated support plate 20 arranged downstream duringfiltering procedure, and a perforated backflushing plate 21 arrangedupstream. Between the two plates 20, 21 is a sieving layer 22 whichexerts the real filtering effect. The two plates 20, 21 serve toabsorbing the pressure exerted by filtered or backflushed melt, and theperforations provided in the two plates 20, 21 are large enough thatthey do not substantially interfere with the flow of melt supplied or ofthe filtrate. The perforated support plate 20 and the perforatedbackflushing plate 21 are arcuate, the convex side being upstream duringfiltering procedure. To each sieving section 16, a connection channel 23is assigned through which melt supplied from the distribution space 7may flow to the corresponding sieving section 16. As is shown in FIG. 2,all connection channels 23 are between both front ends of thedistribution space 7 defined by the cylindrical portions 10, 11 of thecontrol body 9, and also between the orifices 55 of the guide channels 6into the distribution space 7 so that all connection channels 23 servingfor filtering are always flown through from the front ends of thedistribution space 7. Under this flow condition, the melt, supplied bythe guide channels 6, passes the cross-pieces 14. This has the advantagethat no dead regions can develop in the distribution space 7 whereinplastic material can dwell for a prolonged period without moving, thusbeing liable to be thermally damaged. In an extreme case, the outermostconnection channels 23, i.e. the connection channels 23 situated in thearray of connection channels 23 at both ends of this array, may beopposite the two guide channels 6 so that these two guide channels 6 arelocated in the region of the outermost connection channels 23.

In filtering position, the control body 9 of the rotary piston 8 iscaused by a device merely schematically illustrated to assume such arotary position that none of the cross-pieces 14, which are offsetrelative to each other in circumferential and longitudinal direction ofthe rotary piston 8, is opposite to one of the connection channels 23.Therefore, the melt to be filtered is able to flow unimpededly from thewhole distribution space 7 into all connection channels 23 and, thus, tothe sieving sections 16 which are behind. The sieving layer 22 of eachsieving section 16 retains impurities, the filtrate reaches a collectionspace 25 behind all sieving sections 16, and flows from it off thehousing 1 through a discharge channel 34 in the direction of arrow 57 toa discharge opening 26.

If, however, one of the sieving sections 16 is to be flushed back, thecontrol body 9 is turned by means of the device 24 in such a manner thatthe cross-piece 14 opposite the sieving section 16 to be flushed back ismoved in front of the associated connection channel 23. The areaengagement of the front surface of the cross-piece 14 of the wall of theborehole 12 acts as a sealing so that the flow of melt to be filtered isblocked for the corresponding connection channel 23. However, thisconnection channel 23 is moved into communication with a flush backchannel 27 passing through the cross-piece 14 and extending radially orobliquely (FIG. 2) within the rotary piston 8. All flush back channels27 discharge into a discharge channel 28, extending centrally along thelongitudinal axis 13 of the rotary piston 8, through which the massflushed back reaches either ambient or a collection chamber not shown inthe direction of arrow 29.

When flushing back a sieving section 16, the conditions mentioned aboveremain unchanged for all other sieving sections, i.e. these othersieving sections 16 will continue to filter, and only for the sievingsection to be flushed back, the associated connection channel 23 will beclosed by the cross-piece situated in front of it with respect to theincoming material to be filtered from the distribution space 7. For thesieving section 16 to be flushed back, purified filtrate from thecollection space 25 flows through the perforate supporting plate 20, andin reverse direction, as compared with the filtering procedure, throughthe sieving layer 22 and entrains therefrom the accumulated impuritieswhich will be discharged from the sieving section 16 flushed backthrough the perforations of the perforated backflushing plate 21 andwill be directed through the respective connection channel 23 into theflush back channel 27 and from it into the discharge channel 28.Therefore, no backflushed material will reach the distribution space 7.

In order to be able to perform the flushing back procedure justdescribed for each individual sieving section 16, independently from theother sieving sections, the cross-pieces 14 at the circumference of therotary piston 8 are offset relative to each other both in longitudinaldirection and in circumferential direction so that one respectivecross-piece 14 may be moved into sealing engagement with that wallportion of the borehole 12 which surrounds the connection channel 23,while the other cross-pieces 14 are far enough from their associatedconnection channels 23 that the flow of the mass to be filtered intothese connection channels 23 is not disturbed.

As shown, the sieve carrier 2 has not necessarily to be moved forbackflushing. This is advantageous, because in this way the sievecarrier does not wear off for backflushing, and no burnt or decomposedmelt particles can possibly reach the already filtered melt through anygap between the housing and the discharge channel in the sieve carrier.However, the sieve carrier 2 may be displaced in direction of itslongitudinal axis 19 by a device 30 merely schematically illustrated, inorder to make individual or all sieving sections 16 in the housing 1accessible so that a change of any sieving section may be carried out ina simple manner. This makes it possible to form the housing for thesieve carrier 2 and the distributor 3 integral, as shown in FIGS. 1 and2. An alternative would be to form the housing in two pieces, theparting line 31 represented in dotted lines in FIG. 1 being arranged insuch a manner that the whole sieving arrangement 17 is accessible for anexchange of any sieving section after having removed the housing 53which contains the distributor 3. Another possibility for arranging sucha parting line, thus constructively separating the two housings 1, 53,is given by the parting line 58, also shown in dotted lines in FIG. 1,which suitably passes through the connection channels 23. In both cases,it is possible to flange the housing of the control body 9 of thedistributor 3 to the housing 1 of the sieve carrier 2. An embodimentwith separated housings 1, 53 has the advantage that the distributor 3and the sieve carrier 2 form constructively independent components andcan be machined or exchanged independently from each other for either anexchange (e.g. when worn off) or a repair. Therefore, the distributorcan form a component which may be obtained separately on the market.

It is suitable to dimension the length of the connection channels 23 asshort as possible which improves the performance of backflushing.

As shown, material flows through all connection channels 23 and thewhole distribution space 7 both when filtering and when flushing back.Therefore, coking or other thermal damage of the material suppliedcannot occur.

The arcuate shape of the sieving arrangement 17, apart from theenlargement of the sieving surface, has also the advantage that a veryhigh pressure can be absorbed during filtering. If desired or required,however, an additional supporting body 32 may be arranged at that sideof the sieving arrangement 17 which is downstream during filtering andmay conveniently be formed so as to be favorable for the flow, e.g. as arib having an orthorhombic cross-section (FIG. 1).

In a likewise manner, it may be suitable in some cases to support theconvex side of the sieving arrangement 17, in addition to the support bythe partitions 18, by transverse ribs 33 in order to better absorb thepressure occurring during backflushing.

The embodiment according to FIGS. 3 and 4 is substantially a duplicationof the construction according to FIGS. 1 and 2. Accordingly, two sievingarrangements 17 are provided which are arranged in common back to backin a single sieve carrier 2, a distributor 3 being assigned to each ofthem. The construction of each of these sieve arrangements 17 and ofeach of these distributors corresponds to the constructions describedwith reference to FIGS. 1 and 2. The stream to each one of bothdistributors 3 flows via two guide channels 6. A discharge channel 34leads from the collection space 25 being in common to both sievingarrangements 17 to a discharge opening 26 which, in contrast to theconstruction according to FIGS. 1 and 2, is situated at the same side ofthe housing 1 as the inlet opening 4. The direction of flow of thematerial to be filtered in the guide channels 6 is represented by arrows35.

It may be seen in FIG. 4 that it is possible to combine the cross-pieces14 of two adjacent back flushing channels 27 to form a singlecross-piece 14′. Furthermore, it is suitable to provide a throttle 36 atthe discharge end of the discharge channel 28 in order to prevent a dropin pressure within the overall system during backflushing. As may beseen, the rotary piston 8 represented at left in FIGS. 3 and 4 is inthat position in which all sieving sections 16 filter, whereas therotary piston 8 represented at right in FIGS. 3 and 4 is in thatposition in which the connection channel 23 represented on the upperside of FIG. 4 is closed for filtering, the sieving section 16 assignedto this connection channel being, however, flushed back, while the massflushed back is directed into the discharge channel 28 through the flushback channel 27 arranged in the cross-piece 14 which is in blockingposition.

The embodiment according to FIGS. 5 and 6, instead of rotary pistons asa control body 9, has sliders 37 displaceable in longitudinal directionof the distribution spaces 7 and connected to tie rods 38 which may bedisplaced to and fro in the direction of double arrows 39 by the devices24. At the side opposite to the tie rods 38, each slider 37 is connectedto a tube forming an elongation 40 of the slider 37 and containing inits interior the discharge channel 28, while being sealingly guided inlongitudinal direction in the borehole 12. Each one of these sliders 37may be displaced into a position in which it is outside the region ofthe connection channels 23, which position is represented for bothsliders 37 in FIG. 5 and corresponds to the filtering position in whichthe influx of the mass to be filtered is unblocked to all sievingsections 16. In the operational position shown in FIG. 6, the right-handslider 37 is also outside the region of the connection channels 23, butthe left-hand slider 37 is in a position in which its flush back channel27 is communicating with the connection channel 23 of the second sievingsection 16 (when counted from the top of FIG. 6). As may be seen fromFIGS. 5 and 6, the orifices 55 of the guide channels 6 into thedistribution space, also in this construction, are outside the region ofthe connection channels 23 and outside the extreme boundary positions ofthe slider 37 to ensure a flow to the connection channels 23 from theexterior and to avoid dead regions.

As may be seen, the slider 37 has only to have a single cross-piece 14for the functions described above. However, the slider 37 may also havetwo or more cross-pieces 14 arranged in an array, their directioncoinciding with the direction of displacement of the slider 37. In thisway, simultaneous backflushing of two or more sieving sections 16 ispossible. Of course, it should be provided that the slider 37 as awhole, i.e. with all its cross-pieces, may then displaced into aposition where it is outside of all connection channels 23 in order toensure a simultaneous filtering position of all sieving sections 16.Moreover, it has to be made sure for a reliable function that the slider37 cannot inadvertently be turned about its longitudinal axis 59. Thiscan be ensured in a simple way by an appropriate cross-section of theelongation 40 and of the borehole 12 which receives this cross-section,e.g. by a rectangular cross-section.

Although the construction according to FIGS. 5 and 6 requires more spacein the direction of double arrow 39, it has the advantage to enable asimpler construction of the control body 9 and, above all, has theadvantage that the number of sieving sections 16 arranged side-by-sidein the array is theoretically unlimited.

The construction, that is only schematically illustrated in FIG. 7, hasalso a rotary piston 8 as a control body 9 which may be turned in thedirection of arrow 41 about its longitudinal axis by the device 24,while its end portions 10, 11 are sealingly supported in the borehole12. For the sake of simplicity, only two of the cross-pieces 14 of thecontrol body 9 are represented of which the lower cross-piece 14 is inflush back position, whereas the upper cross-piece 14 unblocks the pathof the mass to be filtered from the distribution space 7 into theopposite connection channel 23. The difference to the constructionsdescribed up to now is that the sieving sections 16 are distributed ontotwo sieving arrangements 17, each of which being in a sieve nest 42, andthe two sieve nests 42 being spaced and offset to each other in thedirection of the longitudinal axis of the distribution space 7. Thefiltrate is discharged from both filtering arrangements 17 through adischarge channel 34 each, these two channels 34 combining in thedischarge opening 26.

In the embodiment according to FIGS. 8 and 9, there is also a rotarypiston 8 as a control body 8 whose construction and arrangement issimilar to that of FIG. 7. The difference to FIG. 7 is, however, thatthe sieve carrier 2 has only a single sieve nest 42 in which a sievingarrangement 17 sub-divided into a plurality of sieving sections 16 islocated. The sieving sections 16 are separated from one another bypartitions 18, thus being separately flown through and flushed back. Thepartitions 18 extend parallel to each other from side wall to side wallof the sieve nest 42 and, at the same time, form a support of thesieving arrangement 17 in backflushing direction. It may be seen thatthe connection channels 23 are suitably arranged in such a manner thatthey are respectively centered with respect to the sieving section 16supplied by the respective connection channel 23 in order to ensure asuniform an influx onto the whole active surface of the sieving section16 as possible.

FIG. 10 shows that the individual sieving sections are not necessarilycircular or formed of circular segments. To the contrary, the inventionoffers the possibility to shape the sieving sections as a rectangle orsquare which results in the advantage of an enlarged active sievingsurface and a more uniform flow through, as compared with circularsegments. The corners of a rectangular or square sieving section may berounded in order to avoid dead angles for the flow.

Of course, in all embodiments, more than one sieve carrier may beprovided, and on each sieve carrier a plurality of sieve nests may bearranged, the number of sieving sections per sieve nest theoreticallybeing also unlimited.

FIG. 11 shows a variant in which two sieving arrangements 17, beingopposite to each other and arranged back to back in the sieve carrier 2,are supplied with the material to be filtered by a distributor 3 incommon via the connection channels 23 and are backflushed over the sameconnection channels 23. As compared to the embodiments described up tonow, this variant has the advantage of a smaller and more space savingconstruction, but requires longer connection channels 23.

FIG. 12 shows the combination of a backflushable filtering apparatus 43according to the invention, and a device 44 for plastifying oragglomerating plastic material. This device 44 comprises a housing 45wherein two screw portions 46, 47 are supported interconnected by stillanother screw portion 48 situated between them and acting as a sealingmeans, the conveying direction of this portion being opposite to theconveying direction of the screw portions 46, 47 indicated by an arrow49. In this way, the thermoplastic material conveyed by the screwportion 46 is forced to flow through an opening of the wall of thehousing 45 in the direction of the arrow 56 to the distributor 3 of thefiltering apparatus 43 via a channel 50. From its discharge opening 26,the filtrate flows in the direction of arrow 52 through a channel 51into another opening of the wall of the housing 45 and flows, thus, backto the screw position 47, postponed in conveying direction, whichconveys the filtered plastic material to an extruder nozzle or any otheroutlet.

The sieve carrier 2 is not necessarily a piston having a circularcross-section. To the contrary, it may in many cases be favorable toform the sieve carrier 2 as a flat slider, e.g. having a rectangularcross-section, which prevents in a simple manner any undesired rotationof the sieve carrier 2 about its longitudinal axis.

Within the embodiment according to FIG. 13, which is similar to that ofFIG. 2, additional guiding channels 16 are provided which branch off thetwo guide channels 6 leading to the front ends 61, 62 of thedistribution space 7 which is disposed between these two front ends 61,62. These additional guide channels 60, suitably, have a smaller crosssection than the guide channels 6, and, of course, they are so disposedthat, when the rotary piston 8 is rotated, there is no direct connectionto the backflushing channels 27. These additional guide channels 60offer an advantage in distributing the supplied melt fluid to thecentrally disposed sieving sections 16 and, in general, with respect toan improvement of the flow characteristics. This embodiment, therefore,is particularly suitable for constructions in which a plurality ofsieving sections 16 are disposed side by side in axial direction of thesieve carrier 2.

The embodiment according to FIG. 14 differs from that according to FIG.1 in that two control bodies 9 are disposed within a common housing 53.An own distribution space 7 is assigned to each one of these controlbodies 9, wherein the construction of each control body 9 corresponds tothe construction described in connection with FIG. 1. Each distributionspace 7 is supplied on its both front ends via the guide channels 6.This embodiment has the advantage of still shorter backflushingdistances for the impurity particles to be flushed off, and further thevolume of clean melt necessary for the backflushing process isdecreased. Within this, the transverse ribs 33 are suitable foradditional subdividing the sieving arrangement 17 into several sievingsections 16 to which the distributors 9 are assigned.

Of course, it is also possible to assign to each control body an ownhousing and, if desired, to supply via more than one inlet opening 4.

1-21. (canceled) 22: Distributor for a backflushable filteringapparatus, comprising a housing (53) and a control body (9) forbackflushing moveable therein, to which the material to be filtered issupplied through an inlet opening (4) and which, in filtering position,unblocks the influx of this material to at least two connection channels(23) which may be interconnected to sieving section (16) of thefiltering apparatus, whereas in a flush back position, the control body(9) interconnects one of these connection channels (23) with a dischargechannel (28) situated in the control body (9), the inlet opening (4)communicating with a distribution space (7) arranged in the housing (53)and whose majority surrounds the control body (9), characterized in thatthe inlet opening (4) communicates with the distribution space (7) viatwo guiding channels (6) which discharge at its front ends (61, 62) intothe distribution space (7) within the region of the outermost connectionchannels (23) or outside the region of all connection channels (23). 23:Distributor according to claim 22, characterized in that the controlbody (9) comprises at least one cross-piece (14) at its circumferencewhich overbridges the distribution space (7) and surrounds a flush backchannel (27) communicating with the discharge channel (28), saidcross-piece sealingly engaging an area of the inner wall of the housing(53). 24: Distributor according to claim 22, characterized in that thecontrol body (9) is a slider (37) displaceable in the housing (53) or isa rotary piston (8) rotatable in the housing (53), said rotary piston(8) comprising at least two cross-pieces (14) which are offset relativeto each other both in axial direction and in circumferential directionof the rotary piston (8). 25: Distributor according to claim 22,characterized in that at least one additional guiding channel (60) leadsinto the distribution space (7) between the two guiding channels (6)leading to its two front ends (61, 62). 26: Distributor according toclaim 22, characterized in that at least two control bodies (9) aredisposed within a common housing (53), wherein a distribution space (7)is assigned to each one of these control bodies (9), which distributionspace is supplied in the region of its front ends (61, 62) via guidingchannels (6).